1. Field of the Invention
The present invention broadly relates to a pneumatic cylinder device and, more particularly, to a pneumatic cylinder device having a decelerating mechanism which retards the piston of the cylinder device during the stroking down to a desired low speed such as to allow the piston to stop at a designated position, thus attaining a high precision of the stopping position control.
2. Description of the Prior Art
Japanese Utility Model Laid-Open No. 108604 discloses a pneumatic cylinder device with a fluid-pressure type braking system capable of stopping the piston at a desired position by making use of a control means such as a three-way valve. More specifically, this pneumatic cylinder device has a ram which is urged by a spring such as to displace a fluid from a booster chamber. The pressure of the displaced fluid is applied through a hydraulic chamber to the outer peripheral surface of a slitted bush on the piston, such as to radially contract the bush thereby braking and stopping the piston rod.
On the other hand, Japanese Patent Publication No. 43762/1982 discloses a pneumatic cylinder device with a mechanical-type locking mechanism. This pneumatic cylinder device has a lock cylinder connected to the pneumatic cylinder and receiving a lock piston urged by a spring. The lock piston includes a fastening member having a wedging function and adapted to press a plurality of balls which in turn press a lock shoe receiving the piston rod, thus braking and locking the piston rod.
When these pneumatic cylinder devices with a piston rod stopping function are used for driving various machines, it is necessary to employ many controlling parts or elements such as solenoid valves, control circuit, and a sensor for detecting the position of the piston rod.
Unfortunately, however, the conventional control systems have not been able to provide sufficiently high precision of stopping position control for the following reasons. For instance, the precision of the stopping position control is impaired by the fluctuation in the response characteristics of the solenoid valves. When a microcomputer is used as the control circuit, the time duration from the moment at which a signal is input from the piston rod position sensor till the moment at which the microcomputer actually delivers the driving signal varies depending on the timing of scanning performed in the microcomputer. In addition, the speed of the piston rod varies momentarily causing a difference in the response to the control.
In order to achieve a higher precision of the stopping position control, it has been proposed that a so-called feedback control be adopted in which a comparison is made between the actual stopping postion of the piston rod and the command stopping position and the control is made to nullify the difference. This feedback control is generally expensive and inherently unstable in operation.